(1) Field of the Invention
The invention relates to the field of leak detection enhancement of small cavity devices which depend on hermetic seal integrity.
(2) Description of Prior Art
There are many materials used in manufacturing today which are deleteriously affected by common environmental ingredients. Some of the manufactured devices are able to survive in the presence of the moisture or oxygen in our air, while others are degraded, impaired, or caused to fail in the performance of their design objective. Items which fall into this category requiring that they be sealed from the atmosphere, are called "Hermetic Devices". Common examples of these are: light bulbs, television tubes, electronic devices (such as integrated circuit chips, microelectronics, semiconductors), firing mechanisms used in military applications, and most explosives used in safety device applications such as aircraft emergency release systems, automotive air bag deployment, and the like. Materials used in these devices are considered to be "Chemically Incompatible" with our normal atmosphere.
Applications of technology which require manufactured devices to be hermetic (i.e., sealed from atmospheric contaminants such as water vapor, oxygen, organic vapors, and the like), impose an additional need for proof that the devices are in fact sealed. Such verification of hermeticity is usually accomplished through leak testing. There are three basic methods of leak testing which are commonly used in industry.
The first of the methods requires immersion of the device in a very hot liquid for bubble testing. If the device has a leak, the hot liquid will cause gases within the device to expand, leak out of the device, and be seen as bubbles coming out of the actual hole, or leak, in the part. It is designed for the detection of large leaks in the package or device. Such leaks are in some cases visible to the naked eye, and would allow very large amounts of contaminate to leak into a device in a few hours, and thus damage it. This method has achieved limited success because: (1) many devices are never allowed to be submerged in hot liquids, due to possible chemical incompatability, or, in the case of explosives, the dangers of their instability at elevated temperatures; (2) in many cases the devices could not be reworked or repaired after having been immersed in a liquid; and (3) bubble testing is of little value to devices which have no internal free void or cavity which would normally provide gas for leaking out of the device as bubbles.
The second method uses helium gas. One common form of this method consists of subjecting a hermetic device to a pressure of helium gas for some period of time to allow helium to leak into the device (if the device has a leak), and then placing the device in a gas analyzer system and measuring the gas which is sucked back out of the device, if gas has previously entered through the leak.
Many difficulties are encountered with this method and its success for small cavity devices is limited because: (1) the helium gas concentration in the part must be known, for accurate results; (2) there must either be a large enough void or free volume in the part to hold the helium which leaked into it, or there must be some medum within the part which will absorb and hold the helium gas, and then release it when the part is being subjected to gas analysis. With no internal void or absorption medium, this method is quite ineffective. One common variation in the application of the tracer gas leak detection method involves the sealing of a tracer gas within the cavity of the device at the time the device is manufactured. If the percentage of tracer gas is known, and the device is tested for leaks immediately after the manufacturing steps are completed, there is some degree of confidence that a leaking device could be detected. As internal cavity sizes get smaller, the confidence that a leaking device could retain sufficient tracer gas for detection is lost. Additionally, with nothing internally retaining the tracer gas, the loss of the tracer gas is known to be rapid. An even more serious problem exists when a user of such a device makes an assumption that there could still be tracer gas within the device for conventional leak testing at some later date.
The third method uses a radioactive gas, such as Krypton-85, for pressurization of the device. If the device leaks, some of the radioactive gas will enter the device. As in the helium method, this method also has limited success for small cavity devices because: (1) there must either be a free void to hold the gas, or an absorption medium must be used to hold the tracer gas for detection. In the radioactive gas case, the radiation from the tracer gas leaking into the device is detected or measured in place within the device, and is not required to be drawn back out of the device for detection, as in the helium test, thus allowing very small quantities or radioactive tracer to be detected. As with the helium method, if there is no internal void or absorption medium, this method is also unreliable.